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KF Titration – Sample processing

In the videos you can see suggestions on how to add liquid, viscous, and solid samples to the Karl Fischer cell.

Video tutorials

Liquid sample

Viscous sample

Solid sample

FAQs

What to do if the drift is too high?

If the drift during pretitration (conditioning) is too high after the addition of fresh working medium, then this is a sign that the titration cell is not protected sufficiently against humidity. Replace the molecular sieve and the septum, also check whether all of the O-rings/PTFE sleeves are in proper working condition (damage, cracking).

If the drift is too high after the completion of a sample titration, then this is usually because of other reasons:

The sample has not dissolved completely during titration and continues to release water. An increase of the extraction time or other sample pretreatment steps are recommended. Read for more details in the following FAQ: «What to do to improve the release of water?»

The sample reacts with the KF reagent, a side reaction takes place. Depending on the type of the side reaction, this can be suppressed not only with special reagents, but also by means of appropriate titration methods (KF oven method, titration in the cold, or external extraction).

The sample has shifted the pH value. Read for more details in the following FAQ: «How can I prevent side reactions or pH value shifts?»

How much sample to weigh in?

Volumetric KF titration

The sample size depends on the water content of the sample. In principle, the sample size should be selected in such a way that the titrant consumption lies between 10% and 90% of the buret volume. This means that if work is being done with a 10 mL Exchange or Dosing Unit, the consumption level of the titrant should be between 1 and 9 mL.

In addition, you should take care to ensure that the sample weight selected is not too low. The weighing error will be too great on the one hand, while on the other the sample will no longer be representative. In cases of high water content, it may be better to use a Dosing or Exchange Unit with a greater buret volume in order to avoid filling the Exchange Unit during the titration.

If the sample does not dissolve completely in the working medium, then the sample size has perhaps been selected too high and exceeds the dissolving capacity of the working medium. This can be remedied through the use of a smaller Dosing/Exchange Unit or a titrant with a lower water equivalent, e.g., 2 mg/mL or 1 mg/mL. Despite the lower initial weight, the titrant consumption will then achieve 10 to 90% of the buret volume.

As a general rule: When the titrant consumption is low, you should work with a lower titer (e.g., Titrant 1, Titrant 2), and with a high titer when titrant consumption is a high (Titrant 5).

Table: Recommended sample size

Sample

Approximate sample size [g] for 5 mL buret

Expected water content

Titrant 1

Titrant 2

Titrant 5

0.5%

0.1–0.9

0.2–1.8

0.5–4.5

1.0%

0.05–0.45

0.1–0.9

0.25–2.25

5.0%

–

0.02–0.18

0.05–0.45

10.0%

–

–

0.025–0.225

25.0%

–

–

–

50.0%

–

–

–

Sample

Approximate sample size [g] for 10 mL buret

Expected water content

Titrant 1

Titrant 2

Titrant 5

0.5%

0.2–1.8

0.4–3.6

–

1.0%

0.1–0.9

0.2–1.8

0.5–4.5

5.0%

0.02–0.18

0.4–0.36

0.1–0.9

10.0%

–

0.02–0.18

0.05–0.45

25.0%

–

–

0.02–0.18

50.0%

–

–

0.02–0.09

Sample

Approximate sample size [g] for 20 mL buret

Expected water content

Titrant 1

Titrant 2

Titrant 5

0.5%

0.4–3.6

–

–

1.0%

0.2–1.8

0.4–3.6

–

5.0%

0.04–0.36

0.8–0.72

0.2–1.8

10.0%

0.02–0.18

0.04–0.36

0.1–0.9

25.0%

–

0.016–0.144

0.04–0.36

50.0%

–

–

0.02–0.18

Coulometric KF titration

The sample size depends on the water content and should be small in order to allow you to titrate as many samples as possible in the same reagent and to keep the determination time short. However, ensure that the sample contains at least 50 µg of H2O.

The following table provides an indication of the sample size.

Water content of the sample

Sample size

H2O, to be determined (water content)

10000 ppm = 1%

10 mg–100 mg

100 µg–1000 µg

1000 ppm = 0.1%

100 mg–1 g

100 µg–1000 µg

100 ppm = 0.01%

1 g

100 µg

10 ppm = 0.001%

5 g

50 µg

What to do to improve the release of water?

In cases of samples with low solubility, one of the following procedures is recommended:

A) Use of a solubility promoter

Volumetric KF titration

Solvent

Percentage

Samples

Formamide

max. 50%

Sugar samples, salts

Chloroform

max. 70%

Oil, grease

Long-chain alcohols

max. 50%

Oil, grease

Xylene, toluene

max. 70%

Crude oils with tars

Coulometric KF titration

Solvent

Percentage

Samples

Formamide*

max. 20%

Sugar samples, salts

Chloroform**

max. 30%

Oil, grease

Long-chain alcohols**

max. 30%

Oil, grease

Xylene, toluene**

max. 30%

Crude oils with tars

* Only with generator electrode without diaphragm.
** Use generator electrode with diaphragm if the amount of solubility promoter is higher than 10%.

B) Titration at a higher temperature

Temperature

Samples

40 °C / 50 °C

Vegetable samples, salts, fat-containing samples, food samples

C) Mechanical pretreatment

Homogenizer

Mortar

Lab mill (cooled)

These procedures can also be combined with each other. Detailed information as to which procedure is suitable for which sample can be found in the Metrohm KF monograph.

The titration does not finish. What could be the cause?

A side reaction is taking place. Read for more details in the following FAQs: «How to recognize a side reaction?», «How to prevent side reactions?»

The cell is not leak-tight. In this case carry out the following measures: replace molecular sieve, O-rings/PTFE sleeves, and septum.

There are deposits on the indicator electrode. Read for more details in the following Installation FAQ: «How to clean the indicator electrode?»

How to recognize a side reaction?

The following characteristics indicate the presence of a side reaction:

High drift after the titration has been completed, slow endpoint recognition, or no endpoint found at all.

The water contents determined are not independent of the sample weight.

The results are too high or too low (depending on the type of side reaction).

The water recovery rate found when spiking samples is not within 100 ±3%.

See Figure 1: Curve for a titration with side reaction.

The red curve shows the typical curve shape for a KF titration with side reaction. The curve exhibits a continuous slight rise after the water has been titrated, meaning that there is a steady consumption of reagent to be observed as a result of the side reaction, which leads to incorrect results.

See Figure 2: Substances that can cause side reactions.

How to prevent side reactions?

Use of so-called K-reagents with samples that contain ketones and aldehydes.

Methanol-free reagents when the sample reacts with the methanol of the reagent.

When the sample causes a pH shift to the acid range, buffering can be accomplished with solvent and imadazole.

When the sample causes a pH shift to the alkali range, adding approx. 5 g salicylic acid or benzoic acid to the working medium helps to adjust the optimum pH range for the KF reaction.

To suppress the side reaction, peroxides can be titrated under cold conditions, e.g., at –20 °C to –60 °C (set Ipol to 5 µA).

If the sample (e.g., motor oil) contains various substances that react directly with the KF reagent, then a KF oven will be of help.

You will find more detailed information as to which procedure is suitable for which sample in the Metrohm KF monograph.

Overtitration took place, i.e., the titrated solution is dark yellow to brown instead of light yellow. How to prevent this?

A. It is usually sufficient to clean the platinum pins of the electrode with a soft paper tissue. However, you should take care to ensure while doing so that the platinum pins are not pressed together. They should always be parallel to one another.

B. In volumetric KF titration, the antidiffusion valve on the buret tip should be checked.

See the photo: good buret tip in comparison with defective buret tips

C. Check if you are using the recommended parameters:

Volumetric KF titration

Ethanol-containing reagents

Methanol-containing reagents

Polarization current Ipol

20 µA

50 µA

Endpoint indication at

500 mV

250 mV

Coulometric KF titration

Ethanol-containing reagents

Methanol-containing reagents

Polarization current Ipol

10 µA

10 µA

Endpoint indication at

20 mV

50 mV

D. A further cause of overtitration could be that the initial solution no longer contains sufficient methanol or ethanol. In such a case it is worthwhile to renew the cell solution and/or to check whether too much solubility promoter has been used. Read for more details in the following FAQ: «How often must the KF working medium be replaced?».

See table: Solubility promoters for volumetric and for coulometric titration

Volumetric KF titration

Solvent

Percentage

Samples

Formamide

max. 50%

Sugar samples, salts

Chloroform

max. 70%

Oil, grease

Long-chain alcohols

max. 50%

Oil, grease

Xylene, toluene

max. 70%

Crude oils with tars

Coulometric KF titration

Solvent

Percentage

Samples

Formamide*

max. 20%

Sugar samples, salts

Chloroform**

max. 30%

Oil, grease

Long-chain alcohols**

max. 30%

Oil, grease

Xylene, toluene**

max. 30%

Crude oils with tars

* Only with generator electrode without diaphragm.
** Use generator electrode with diaphragm if the amount of solubility promoter is higher than 10%.

Additional measures:

E. Increasing the stirring rate.
F. Check whether the platinum pins of the electrode are immersed completely in the cell solution.
G. Check whether the correct reagents were combined: if the titrant of the two-component reagent is used with methanol, the KF reaction cannot take place, because no SO2 is available for the reaction. (Only volumetric KF titration.)

The volumetric Karl Fischer titration is very slow

In the case of two-component-reagents, the water capacity is limited by the amount of SO2 and buffer in the solvent. Decreasing pH values and decreasing SO2 concentrations result in a drop in titration speed.

Reagent manufacturers have published the following guideline: In the case of cell solutions with 20 to 25 mL of solvent, the cell solution should be replaced after approximately 30 mL of titrant has been used.

Poor reproducibility of the results

The following causes can be responsible for poor reproducibility:

Cause

Remedy

Sample amount too small (weighing error)

Increase the sample amount. If samples with high water contents are analyzed via volumetric Karl Fischer titration it may be better to use a Dosing or Exchange Unit with a greater buret volume in order to avoid filling of the Dosing or Exchange Unit during the titration.

Insufficient use of titrant (volumetric KF titration)

Increase the sample size and select an Exchange or Dosing Unit so that at least 10% of the buret volume is used, or use a titrant with a lower titer.

Inhomogeneous sample

Increase sample amount.

Homogenize sample prior to determination.

Sample does not dissolve completely

Add smaller amounts of sample or use solubility promoters.

Sample remains on the wall of the vessel

Slight shaking of the titration vessel.

Overtitration

Read for more details in the following FAQ: «Overtitration took place, i.e., the titrated solution is dark yellow to brown instead of light yellow. How to prevent this?»

Why are the values found too high?

Results may be too high for the following reasons:

Cause

Remedy

Side reaction

Read for more details in the following FAQ: «How to recognize a side reaction? How to prevent side reactions?»

The sample is hygroscopic

Here we recommend optimizing the sampling and the sample storage as well as working speedily during the determination.

The sample is inhomogeneous

Increase the sample size.

Homogenize the sample prior to the determination.

Overtitration

Read for more details in the following FAQ: «Overtitration took place, i.e., the titrated solution is dark yellow to brown instead of light yellow. How to prevent this?»

Only for volumetric KF titration:

The titer has changed since the last determination

Redetermine the titer.

Air bubbles in the tubing

Flush the tubing system using the Preparation/«Prep» command.

Non-leak-tight tubing system

Check for crystals in the tubing system.

Check whether tubing is connected tightly.

Check tubing connections.

Why are the values found too low?

Results may be too low for the following reasons:

Cause

Remedy

Side reaction

Read for more details in the following FAQ: «How to recognize a side reaction?», «How to prevent side reactions?»

The sample is inhomogeneous

Increase the sample size.

Homogenize the sample prior to the determination.

The sample dissolves incompletely

Read for more details in the following FAQ: «What can I do to improve the release of water?»

The pretreatment of the sample is not optimal.

Read for more details in the following FAQ: «What to do to improve the release of water?»

Titer is incorrect (volumetric KF titration)

Redetermine the titer with certified standards.

Titration stops too soon

Reduce Stop Drift or work with a relative Start Drift.

The sample gives off water even during sample preparation

Here we recommend optimizing the sampling and sample storage, as well as rapid working during the determination sequence.

Trend within the results

One reason for decreasing water contents in a series may be that the solvent is exhausted and must be replaced. Decreasing water contents within a series can also occur with samples which give off water during sample handling. Here we recommend optimizing the sampling and sample storage, as well as rapid working during the determination sequence.

A further possible reason for apparently decreasing water contents within a series in volumetric Karl Fischer titration is that the buret has not been prepared. The titrant in the cylinder and in the tubing is exposed to humidity, which can result in increased titrant consumption at the beginning of a series.
Increasing water contents within a series can occur with hygroscopic samples. Here we recommend optimizing the sampling and sample storage, as well as rapid working during the determination sequence.

Tip: You can recognize very quickly whether your sample is either releasing water, is hygroscopic, or unproblematic by determining the water content of the sample immediately and then leaving the sample to stand in a beaker in the laboratory air and determining the water content again after 10, 30, and 60 minutes.

How often is the KF working medium to be changed?

In the case of two-component-reagents, the water capacity is limited by the amount of SO2 and buffer in the solvent. Decreasing pH values and decreasing SO2 concentrations result in a drop in titration speed.

Reagent manufacturers have published the following guideline: In the case of cell solutions with 20 to 25 mL of solvent, the cell solution should be replaced after approximately 30 mL of titrant has been used.